Effect of transparent substrate on properties of CuInSe2 thin films prepared by chemical spray pyrolysis

In this paper, the properties of CuInSe2 (CISe) films deposited on three transparent substrates (FTO, FTO/NiOx, FTO/MoO3) are studied. These substrates might be used for bifacial solar cells, in place of the conventional glass/Mo substrates. CISe layers are deposited by spray pyrolysis followed by a selenization process. For the same deposition conditions, the CISe layers on FTO show the largest grain size (~ 0.50 µm) and crystallinity, while FTO/MoO3 substrates result in the smallest grains (~ 0.15 µm). The optical bandgap of the CISe films ranged from 1.35 eV for FTO substrate to 1.44 eV for FTO/MoO3 substrate. All films show p-type conductivity, with the carrier densities of 1.6 × 1017 cm−3, 5.4 × 1017 cm−3, and 2.4 × 1019 cm−3 for FTO, FTO/NiOx, and FTO/MoO3 substrates, respectively. The CISe films also show different conduction, and valence levels, based on the substrate. In all cases, an ohmic behavior is observed between the CISe and substrate. The results demonstrate that CISe layer crystallinity, carrier concentration, mobility, and energy levels are strongly dependent on the chemical nature of the substrate. Bare FTO shows the most appropriate performance in terms of device requirements.


Scientific Reports
| (2022) 12:14715 | https://doi.org/10.1038/s41598-022-18579-w www.nature.com/scientificreports/ powdery film formation at high temperature/low deposition rate conditions. The desired deposition regime lies between these two extremes. The solution was sprayed by air as carrier gas and using a conventional airbrush. Conditions during selenization have an extreme influence on crystallization and large grain formation. The selenization process was done for ten 1.4 × 1.4 cm 2 films in a graphite box with elemental Se pellets. Before starting the selenization process, the furnace tube was first purged with nitrogen gas and then was set at a predetermined pressure of 500 Torr. The heating profile lasted for 40 min, containing ramping up (~ 25°C min −1 ) to 500 °C for 20 min and was resided for 20 min. For ease of reference, the films formed on different substrates are named as Characterization. The morphology, composition and crystal structure of different CIS(e) films before and after selenization were examined by high-resolution field emission scanning electron microscopy (FESEM; HRSEM, XL30SFEG Phillips Co., Holland at 10 kV), energy dispersive spectroscopy (EDS; EDAX Genesis apex, acceleration voltage: 30 kV). To measure the roughness, atomic force microscopy (AFM) (VEECO-CP research) was used with a silicon tip of 10 nm radius in tapping mode. The crystal structure of the as-sprayed thin films was analyzed by X-ray diffraction (XRD) technique (X'Pert Pro MPD, PANalytical) with CuKα (λ = 1.5406 Å) radiation in the 2θ range from 4° to 80°. The scanning mode is continuous with a step size of 0.02°and scan step time of 0.5 s. The optical properties of the deposited layers were evaluated by measuring the transmittance spectra by Ultraviolet-Visible (UV/Vis) spectroscopy (Lamda 25, Perkin Elmer). The Mott-Schottky (MS) analysis was performed in a three-electrode system, in 0.5 M Na 2 SO 4 solution (pH 6.0) as an electrolyte using an EIS-26H system (IRASOL). The working, reference and counter electrodes were (FTO, FTO/NiO x , FTO/MoO 3 ), Ag/AgCl (3 M KCl), and Pt rod, respectively. The frequency of the signal was 1 kHz, and the bias voltage was scanned from − 0.8 V to 0.3 V, with 50 mV s −1 speed (peak-to-peak) at ambient conditions. All experiments proceeded after 5 s electrode stabilization. Charge mobility of CISe films was measured using Keithley 2400 Source meter.

Results and discussion
Morphological and structural properties. The Fig. S1. Figure 3 represents the FESEM surface and cross-sectional images of CISe thin films on different substrates. As shown in FESEM surface images (Fig. 3a-c), all deposited films show dense and crack-free surface morphology while the FTO films show larger grains compared to other films (Fig. 3a). The largest grain size was calculated using ImageJ software which was estimated to be ~ 0.50 μm ( Table 1). The FESEM surface images of CISe films are also shown in smaller magnifications in Supporting Information, Fig. S2.
Cross-sectional FESEM images of CISe films on various substrates have been shown in Fig. 3d-f. The morphology of deposited CISe films on the FTO substrate looks very similar to the case of CISe growth on FTO/ NiO x substrate, hence the thickness of the film appears to be affected by the type of substrate. The deposited CISe films on the FTO substrate have the highest roughness substrate and film thickness is about 2.3 µm (Fig. 3b),   (Fig. 3e). On the other hand, the CISe films grown on FTO/MoO 3 substrate show a bilayer structure in which the small and large grains are placed at the bottom and the top surface, respectively. As shown in Fig. 3f pores in the film can act as pathways for the evaporation of volatile materials (such as Cl 2 or In 2 Se) during the selenization process 57 . Similar results were previously reported for doctor blade coating of CIS precursor solutions on Mo substrates 58 , solution-processing of amorphous nanoparticle-based CISe films on the Mo substrate 59,60 , which selenization of the CIS films with Se vapor at high-temperature results in bi-layered films with an upper layer of chalcopyrite CISe and a small grain-sized bottom layer.
The exact influence of this bilayer on different film properties and the resulting device is not still clear, and contradictory views are present. Generally, the generated bi-layered CISe films have been attributed to the presence of carbon at the bottom 58-60 , formation of CIS on top of the layer and hinder further evaporation of solvent 58 , the existence of mixed-phases, consisting of the ordered vacancy compound (OVC), Cu-Se phases, CISe and trace of CIS in the top layers 59 . In this study, the probable cause for creating a bilayer structure for CISe film growth on FTO/MoO 3 substrate is the existence of impurity.
The composition of each sample was determined by EDS analysis (see Fig. S3 and Table S1). The compositional ratio of CISe films deposited on FTO, FTO/NiO x , and FTO/MoO 3 substrates showed that the S/In ratio was   Also, the CISe films have a Cu/In ratio in the range of 0.77-1.04. The atomic ratio of Cu/In in the CISe film on the FTO substrate is larger than that of the initial solution before depositing. These results revealed that the films changed from the In-rich CISe phase, in the FTO/NiO x and FTO/MoO 3 films to the stoichiometric CISe phase. This phenomenon can be described by the formation of volatile In 2 Se during the selenization process and subsequent evaporation due to the high vapor pressure of In 2 Se 61,62 . As confirmed by EDS results, there is a minor Cl residue (0.08 at%) in the FTO films and a major chlorine (Cl) residue (0.50 at%) in the FTO/MoO 3 films within the EDS resolution. The high chlorine residual content indicates an incomplete reaction between the precursor ingredients in the FTO/MoO 3 films and can lead to the formation of a bilayer structure in these films. These results are in good agreement with the results of the FESEM-analysis. Figure 4 represents the XRD patterns obtained from the CuInSe 2 films used for structural and materials identification study. It can be observed that three prominent peaks identified as the planes of (112), (204), and (215) corresponding to CuInSe 2 chalcopyrite tetragonal crystal structure and are in good agreement with the standard JCPDS file (standard JCPDS no. 085-1575 and 01-075-0107) for CIS and CISe films, respectively. In addition, no peaks of other impurities such as Cu x Se, In 2 S 3 , etc. were detected, indicating the high phase purity of CuInSe 2 films (Fig. 4a). However, considering the used CISe reference card, a noticeable peak shift towards higher diffraction angles can be observed considering the reflections of the detected chalcopyrite phase. This peak shift can be described by a non-complete substitution of sulfur with selenium. The smaller atomic radius of the remaining sulfur compared to selenium results in the smaller unit cell of the chalcopyrite phase, which causes a peak shift towards higher diffraction angles and forms the CI(S, Se) alloy (Fig. 4b) 63 . Figure S4 demonstrates the XRD pattern obtained from the NiO x and MoO 3 films compared to standard prominent peaks identified as the planes of (1 1 1), (2 0 0), and (2 2 0) demonstrating a cubic crystal structure for NiO x thin films and planes of (0 2 0), (1 2 1) and (1 5 0) showing an orthorhombic crystal structure for MoO 3 which is in good agreement with the standard JCPDS file (standard JCPDS no. 00-002-0422 and 01-076-1003) for NiO x and MoO 3 films, respectively.
To research, the structural properties of CISe films such as dislocation densities, micro-strain, the number of crystallites per unit area, etc. have been calculated from the major (112) peak of X-ray microbeam studies. The dislocation density (δ) presents information about the crystal structure of CISe films, which can be evaluated using Williamson and Smallman's equation (Eq. 1) 64 : where D is the crystallite size.
The micro-strain (ε) influences the optoelectronic properties of the CISe thin films due to the distorted lattice. The average micro-strain present in the CISe films was calculated by Eq. (2) 65 : where β is the full peak width at half maximum (FWHM)and θ is the Bragg angle.
Furthermore, the number of crystallites per unit area (N) was estimated by using the following equation 66 : where (t) is the thickness of the film. As reported in Table 2, the dislocation density of films is found to increase from 4.52 to 44.44 [(lines m −2 ) × 10 −2 ] with changes in the substrate from FTO to FTO/MoO 3 . This indicates that the crystallinity of the  The minimum amount of dislocation density and the micro-strain of the CISe films were 4.52 × 10 14 and 4.45 × 10 -2 cm −2 , respectively. These values are significantly lower than the spray pyrolyzed CuInGaS 2 and CuAlS 2 films in literature 68,69 . The reduction in the dislocation density and micro-strain for FTO substrate was most probably due to the stress relaxation, which occurs during the recrystallization process 69 .
Also, the number of crystallites per unit area significantly changes with the substrate type. The number of grains increased notably up to about 622 × 10 8 cm −2 for FTO/MoO 3 substrate, ( This result is probably due to the highly increased growth rate in FTO/MoO 3 substrate, which results in lower grain size and a higher number of crystallites per unit area in the films 68,69 . Optical properties. The optical transmittance spectra of CISe films by various substrates within the range of 350-1100 nm are shown in Fig. 5a. UV-Vis transmittance spectra can be used to extract the band gap energy of the films using the Tauc plot formalism 70 : where A is a constant, n = 0.5 for allowed direct band transition, h is the Planck constant, α is the absorption coefficient near the absorption edge and E g is the optical band gap value. The optical bandgap energy of the CISe films has shown in Fig. 5b. Moreover, the optical transmittance slightly decreased with depositing CISe films on FTO. The transmittance of this film was approximately 40%. Figure 5a shows the absorption edge shifts to shorter wavelengths with the variation of the substrate type from FTO to FTO/NiO x and FTO/MoO 3 for films. From the Tauc plot analysis (Fig. 5b), energy band gap values of 1.35, 1.41, and 1.44 eV were found for FTO, FTO/NiO x , and FTO/MoO 3 CISe films, respectively (Table 3).    71 , as shown by the EDS and XRD results. Also, a further decrease in band gap value for FTO films may be due to crystallinity improvement [75][76][77] . The band gaps are higher for FTO/NiO x and FTO/MoO 3 films with smaller crystalline sizes, which may be due to the density of states at the interfaces, grain boundaries, and the defects energy level on the surface 78,79 .

Electrochemical properties
To further investigate the effect of the substrate on the electrochemical properties of CISe films, we have measured the Mott Schottky (M-S) relationship based on the capacitance versus applied potential. The Mott Schottky equation is given as follows 80 : where C sc is the space charge capacitance, ε r is the dielectric constant of the CISe film (13.6) 81 , ε 0 is the permittivity of a vacuum, e is the electron charge, A is the film surface area in contact with the electrolyte, N A is the density of acceptor in the semiconductor, V is the externally applied potential, V fb is the flat band potential, k the Boltzmann constant (1.38 × 10 −23 J K −1 ) and T the operation temperature (300 K). The negative slope in Fig. 6 (M-S plots) indicates that all CISe thin films are p-type semiconductors.
The carrier density (N A ) can be also conveniently found by determining the slope of the linear region of the M-S plot by using Eq. (5) 77 . The semiconductor parameters such as values of the V fb , the carrier density N A width of the space-charge region (SCR), W, and energy level have been shown (  85,88,89 . In this work, the high values of carrier density for FTO/MoO 3 films could be due to the presence of more grain borders and grain boundaries 85 , roughness and non-planar interfaces on the surface 85,90 , impurities like Cl 91 , as evidenced by the FESEM and EDS analysis. An important parameter in solar cells or other electronic devices is V fb , which controls the band alignments and carrier transfer at the interfaces 92 . The flat-band potential of the semiconductor can be calculated by intersecting the V-axis of the linear region of the M-S plot 77 . The V fb value shifts significantly from 0.13 to −0.37 V (vs. Ag/ AgCl) with a variation of the substrate type from FTO to FTO/NiO x and FTO/MoO 3 films. This shift can be related to the change in the morphology and the composition of elements by changing the substrate type 77 . The V fb value for CISe films with FTO substrate is more positive than those obtained by other films, that is indicating the better conductivity of FTO thin films due to an increase in their crystallinity 93 which is confirmed by FESEM data.
The width of the SCR, W, is directly related to the capacitance of the CISe films. Equation (6) gives 94 :    80 . Utilizing the other substrate seems quite to reduce the SCR width in the CISe films (to about 4.81 nm), which leads to the limited short-circuit current density (J sc ) values obtained in solar cells 95 . This would prove that the electrically active region of the CISe films is different for the various substrates.

Electrical properties.
To understand more deeply the effect of the substrate type on the hole mobility, conductivity, bulk electrical resistivity, diffusion coefficient, and electrical behavior in the absorber layers, the current density-voltage (J-V) characteristics were recorded in the dark conditions and at ambient temperature. Figure 7 describes the typical curves of current density (J) as a function of the applied potential (V) for FTO, FTO/NiO x , and FTO/MoO 3 films. For all films, the dark J-V analysis indicates a linear characteristic that means good ohmic contacts without an intermediate layer of Mo between CISe films and substrate.
The devices were fabricated with the structure of FTO/CISe/Graphite. Then, the following equation is used to calculate the mobility in the ohmic region 96 : where J is the current density, N A is the carrier density, e is the electronic charge, μ is the hole mobility, V is the applied voltage and d is the distance between the electrodes, (the thickness of the thin film). The electrical parameters of all corresponding films were summarized in Table 5.
Hole mobility values of 7.37 × 10 -2 , 2.08 × 10 -2 , and 1.17 × 10 −3 cm 2 V −1 s −1 were obtained for FTO, FTO/NiO x , and FTO/MoO 3 films, respectively. The measured hole mobility values for FTO and FTO/NiO x films are about one order of magnitude larger than FTO/MoO 3 film, which indicates better crystallinity, uniformity, and grain boundary continuities in these thin films. On the other hand, the larger grain size in FTO and FTO/NiO x films is desirable as it leads to less grain boundary scattering of the charge carriers, i.e., better electrical transport properties 97 . Although the hole mobility values for FTO and FTO/NiO x films are slightly lower than the previously reported data 88,98 . The low value of mobility in FTO/MoO 3 films may be attributed to impurities (i.e. 0.50 atomic% chlorine), which can act as dopants or cause traps that increase recombination or reduce mobility. Moreover, because FTO/MoO 3 films have poor crystallinity and more grain boundary scattering, grain boundary discontinuities and presence of surface states 99 specific surface area and a border effect may intensify carrier scattering at the surface and reduce mobility 85 .
Both the carrier density and the hole mobility contribute to the bulk electrical resistivity and conductivity. The conductivity of the CISe thin films is proportional to the carrier density and hole mobility 100 :  where σ is conductivity, e is the electronic charge, N A is carrier density and µ is hole mobility. The bulk electrical resistivity values were calculated using the following well-known equation 101 : where ρ is bulk electrical resistivity and σ is conductivity. Table 5 shows the substrate type dependence of the bulk resistivity and conductivity of CISe films. The bulk resistivity of the CISe films was about 5.6 × 10 2 to 2.2 × 10 2 Ωcm. These values are similar to the reported values which are in the range 4.3 × 10 2 -5.3 × 10 2 Ωcm 102,103 . Also, the conductivity of all films was between 1.78 × 10 -3 to 4.49 × 10 -3 S cm −1 .
The charge carrier diffusion length in a semiconductor is described by the average distance that charge carriers travel in a semiconductor. The diffusion coefficient and mobility of charge carriers are related by Einstein's equation 104 : where D is diffusion coefficient, µ is hole mobility, k is the Boltzmann constant (1.38 × 10 −23 J K −1 ), T is the operation temperature (300 K) and e is the electronic charge.
The CISe films prepared using various substrates show hole diffusion coefficients from 10 -3 to 10 -5 cm 2 s −1 . The J-V dark measurements results reveal that the hole diffusion coefficient of the FTO films is 1.91 × 10 −3 cm 2 s −1 , which is higher than the FTO/NiO x (5.38 × 10 −4 cm 2 s −1 ) and FTO/MoO 3 (3.03 × 10 −5 cm 2 s −1 ) films. The higher hole diffusion coefficient value is favorable for fast charge transport and results from the effective connection of the grains to create the charge carrier's continuous pathway in the CISe films 105 . However, the existence of deep levels in the CISe films is unsuitable since they act as recombination centers for charge carriers and therefore reduce carrier diffusion coefficients in FTO/NiO x and FTO/MoO 3 films 103 .

Conclusion
In this research, transparent substrates for the replacement of molybdenum (Mo) opaque substrate were studied for use in bifacial photovoltaic devices. Three transparent substrates (FTO, FTO/NiO x , and FTO/MoO 3 ) were used as substrates, and CISe thin films were deposited by spray pyrolysis and selenization. The results of different characterization techniques have a good correlation to each other. The optical transmittance and significant band gap energy of CISe films were changed from 1.35 to 1.44 eV depending on the substrate type. The CISe films deposited on the FTO substrate were more compact, thicker, with larger grains than others. The XRD peaks confirm that all films show a chalcopyrite tetragonal structure without any impurity phase but structural parameters such as micro-strain(ε) of ~ 4.45 × 10 -2 , number of crystallites per unit area (N) of ~ 22.15 × 10 8 cm −2 and dislocation density (δ) of ~ 4.52 (lines cm −2 ) × 10 8 have the lowest values for CISe films on FTO substrates. All CISe films are p-type semiconductors with a carrier density of ~ 10 17 to 10 19 cm −3 . Flat band potential and space-charge layer values of the CISe films are estimated based on the Mott-Schottky analysis to be 0.13 V (vs. Ag/AgCl) and 34.94 nm for FTO substrates, respectively. Dark J-V measurement exhibited that the CISe films have ohmic behavior with a favorable hole mobility of around 7.37 × 10 −2 cm 2 V −1 s −1 and diffusion coefficient of 1.91 × 10 −3 cm 2 s −1 for CISe film deposited on FTO substrate which is notably higher than the other two films. Generally, the optical, physical, and electrical properties of CISe films are influenced by substrate type. It is thought that the properties of CISe thin films deposited on the FTO substrates are considerably close to the properties essential for photovoltaic applications, thus FTO can be an alternative substrate to opaque substrates with deposited CISe films as absorber layer, hole transport layer, and photoanode used in applications such as bifacial and tandem solar cell, supercapacitor and sensor.

Data availability
All data generated or analysed during this study are included in this published article [and its supplementary information files].